Indeed there are rules. These are largely obsolescent. An esquire was at first “a young man of gentle birth, who, as an aspirant to knighthood, attended upon a knight, carried his shield, and rendered him other services”. It became also “a man belonging to the higher order of English gentry, ranking immediately below a knight”. Several classes of men became entitled to be called esquires — younger sons of peers, eldest sons of knights; judges, barristers-at-law, and many others. By now Esq has lost all sense of rank, and can be attached (in correspondence) to the name of any adult male. In the US, Esq is often used by lawyers, both male and female, when referring to or addressing one another in writing. The suffix is dying, let it die.